“Battery capacity” is a measure (typically in ampere hours) of the charge stored by the battery, and is determined by the mass of active material contained in the battery. The present battery capacity (i.e., relative state of charge) can also be expressed as a percentage of the maximum capacity of the battery. The maximum battery capacity represents the maximum amount of energy that can be extracted from the battery under certain specified conditions. The actual energy storage capabilities of the battery, however, can vary significantly from the “nominal” rated capacity.
The battery capacity depends, in part, on the age of the battery. As the battery ages, the internal resistance of the battery increases, thereby affecting the battery capacity.
The charging/discharging rates also affect the rated battery capacity. If the battery is being discharged very quickly (i.e., the discharge current is high), then the amount of energy that can be extracted from the battery is reduced and the battery capacity is lower. This is because the necessary components for the reaction do not have enough time to move to their necessary positions. Only a fraction of the total reactants are converted to other forms, and therefore reducing the available energy. Alternatively, when the battery is discharged at a very slow rate using a low current, more energy can be extracted from the battery and the battery capacity is higher.
The temperature of a battery directly affects the internal resistance of the battery and the energy that can be extracted from it. At higher temperatures, the battery capacity is typically higher than at lower temperatures.
Conventional methods for measuring the battery capacity are prone to errors since they require a known starting point. Conventional methods also do not account for how aging affects the internal resistance of the battery over time, and thus its ability to retain charge.